The invention relates to high thermal stability zirconium phosphates, having the NZP structure. More particularly, the invention relates to transition metal, zirconium phosphates having high thermal stability.
The term NZP refers to NaZr2(PO4)3, as well as to related isostructural phosphates and silicophosphates having a similar crystal structure. The NZP crystal structure is basically a three-dimensional network of PO4 tetrahedra and ZrO6 octahedra which are linked together by shared oxygens. Each zirconium atom is located on a three-fold axis parallel to the c-axis, and is coordinated to six oxygens. Each phosphorous atom is located on a two-fold axis parallel to the a-axis, and is coordinated to 4 oxygens. Thus each ZrO6 octahedron is connected to six PO4 tetrahedra through the six oxygen atoms making up the corners of the ZrO6 octahedron. Sodium ions are located at the interstitial sites created by the framework, but are replaced with other ions depending on the NZP analog. The NZP structure is exceptionally flexible towards ionic substitution at various lattice sites.
The stoichiometry of NZP compounds can be generalized by the formula A0-4B2(XO4)3, wherein A represents a relatively large monovalent or divalent cation, B designates a medium-sized (octahedrally-coordinated) trivalent, tetravalent, or pentavalent cation, and X denotes a small (tetrahedrally-coordinated) tetravalent, pentavalent, or hexavalent cation.
NZP stoichiometry requires the cation ratio B:X to be approximately 2:3. The number of A atoms can range from 0-4, depending upon the net charge of the B and X cations.
A wide variety of NZP type compounds have been prepared. NZP type compounds have been reported for alkali metal and alkaline earth metal cations substituted into the sodium site of the NZP structure. Compounds in which substitutions in the Zr sites, and in the P sites, as well as coupled substitutions involving two or all three of these sites, also have been reported.
Various ceramic compounds have been employed as colorants in ceramic compositions such as glazes. A well known ceramic compound that has been used as a colorant is Zircon. Zircon, however, is expensive. Although Zircon has been widely accepted as a ceramic colorant, a need continues to exist for cost-effective, high thermal stability compounds which can impart desired color characteristics to the ceramic composition during firing.
The invention relates to novel transition metal NZP compounds and diphasic mixtures thereof. These compounds exhibit very high thermal stability and retain their intrinsic color when employed in a ceramic composition.
In a first aspect, inventive compositions have the general formula MxZr4P6O24, where M is any of Mn, Fe, Co, Ni, or Cu, and x is about 0.1 to 1.2, preferably about 0.9 to about 1.0.
In another aspect, The inventive compositions have the general formula (N1xe2x88x92x Mx)Zr4P6O24, where M and N are different and each is any of Mn, Fe, Co, Ni, or Cu, and x is about 0.1 to 1.0, preferably about 0.8 to about 1.0.
The compounds of the invention, due in part to their extremely high thermal stability when for example, employed as additives in ceramic compositions such as glasses and glaze compositions, impart their intrinsic color to the ceramic compositions.
The starting materials used to synthesize the NZP compounds of the invention preferably are chemically pure. However, minor amounts of impurities which do not cause a significantly large amount, i.e. greater than about 3.0% by volume, of any second phase to occur in the compound may be tolerated.
Starting materials which may be used to synthesize the NZP compounds include water soluble salts of zirconium such as zirconium oxychloride and zirconium nitrate, preferably zirconium oxychloride; metal chlorides and metal nitrates where the metal cation is a transition metal cation such as Cu, Fe, Ni, Mn, and Co, for substitution into the N site of the NZP compound; and phosphate compounds such as ammonium dihydrogen phosphate, phosphoric acid, and diammonium hydrogen phosphate, preferably phosphoric acid.
Silicon can be partially substituted for phosphorous, and any deficient charge can be compensated by the transition metal in the N site. Useful sources of silicon include SiCl4, SiO2, tetraethoxysilane, and tetramethoxysilane, preferably tetraethoxysilane.
The NZP compounds of the invention are made from precursors of the compounds. The precursors may be made by various known ceramic processing methods such as solid state reaction and Xerogel. Preferably, the precursors are made by the Xerogel method.
Where the Xerogel method is employed to manufacture the precursors for the NZP compounds any of aqueous metal chlorides, metal nitrates, or mixtures thereof, preferably aqueous metal chlorides, more preferably aqueous metal chlorides having a concentration of about 0.1M to about 2.0M, most preferably about 1.0M to about 2.0M, are combined with any of zirconium oxychloride, zirconium nitrate, and zirconium dioxide, preferably zirconium oxychloride solution having a concentration of about 0.2M to about 2.0M, preferably about 1.0M to about 2.0M to form a mixed transition metal cation solution. The resulting solution is treated with dropwise addition of an aqueous phosphate such as ammonium dihydrogen phosphate, phosphoric acid, and diammonium hydrogen phosphate, preferably phosphoric acid solutions having a concentration of about 0.2M to about 6.0M, preferably about 3.0M to about 6.0M, under constant rapid stirring by a magnetic stirrer to precipitate a gel. In a preferred aspect, the metal chloride, zirconium oxychloride and phosphoric acid are used in amounts sufficient to achieve a ratio of N:Zr:P of about (0.8-1.2):2:3, more preferably about 1:2:3. The resulting gels are dried under conditions sufficient to remove volatiles such as water, hydrogen chlorides, and the like. Typically, the gels are dried in air at about 60xc2x0 C. to about 120xc2x0 C. for about 4 hours to about 24 hours, preferably about 70xc2x0 C. for about 24 hours, and then at about 130xc2x0 C. to about 175xc2x0 C. for about 4 hours to about 24 hours, preferably about 150xc2x0 C, for about 4 hours to produce a powder. The powder is fired at about 800xc2x0 C. to about 1100xc2x0 C. for about 4 hours to about 16 hours. The powder may be fired as loose powder, or may be compressed at about 100-200 MPa into pellets prior to firing.
The novel transition metal NZP compounds of the invention have extremely high thermal stability over a wide range of temperatures. These compounds may be used for a variety of applications including, but not limited to, colorants in glazes, colorants in glasses, colorants in polymers, ceramic catalytic convertors, emission control devices, space optics, and the like.